The present invention refers to methods and devices for robot-aided laser welding, namely for welding metal sheet structures, for instance structures constituting assemblies or subassemblies of motor-vehicle bodies or frames, or structures not related to the automotive field.
Long ago the Applicant proposed (see for instance European patents no. EP 0 440 001 B1 and no. EP 0 440 002 B1 related to the so-called “LASERGATE” system) devices for laser welding of motor-vehicle structures. The use of laser welding for such applications, however, has not spread much straightly after its first proposal at the beginning of the '90s. This is mainly due to the fact that tests made with the first embodiments of laser welding devices have shown the presence of several problems related to such technology.
A first relevant problem is a result of the wide use in the automotive field of metal sheets provided with an outer zinc-plated protection layer. Said layer gives rise to the generation of zinc vapors during laser welding, which often jeopardize a good quality welding.
Such problem has been dealt with and disclosed in detail in European patent applications no. EP 1 238 748 A1 and no. EP 1 236 535 A1 belonging to the same Applicant, which relate to devices that can overcome in a simple and efficient manner the aforesaid technical hindrance, ensuring outlets for zinc vapors generated during the welding process. Another solution to the problem of zinc vapors has also been suggested in the Italian patent application TO2002A000760 belonging to the Applicant.
Some time had to lapse, however, before the aforesaid technical problem was acknowledged, studied and solved completely, which explains at least partly the long period in which the use of laser welding was still in progress in the automotive field.
Another relevant problem to be kept into account in the application of laser welding for assembling motor-vehicle structures is related to the need for a high assembly quality with short manufacturing times. An assembly station for a motor-vehicle body or for a subassembly thereof typically comprises a plurality of locating and blocking systems ensuring the correct position of the metal sheet elements of the structure during welding. There is obviously a minimum threshold for the number of blocking systems that can be arranged to said purposes, below which the structure geometry is not conveniently ensured, thus resulting in a low-quality assembly operation. This gives rise to a relatively “crowded” welding station, full of blocking systems with their control devices for switching them from an open rest condition to a closed operating condition and vice versa. More to the point, in case of flexible welding stations, i.e. stations operating on several types or models or versions of structure to be welded, the welding station is also equipped with guiding and controlling means for several structures supporting the blocking systems, rapidly interchangeable depending on the type of body or subassembly each time getting into the welding station. The subsequent relative complexity of the architecture of the welding station and of its parts obviously complicates the job of handling robots used for bringing welding means (electric welding guns in conventional cases, laser heads for laser welding) close to the various areas of the structure to be welded.
Both in the case of traditional techniques with electric welding guns and of laser welding, robots should successively reach a series of areas of the structure to be welded in order to carry out the welding operations assigned to them. Therefore, after the structure to be welded has reached the welding station, it should stay in said station for a period at least long enough to enable every robot to carry out all the operations assigned to it. Obviously, the period spent in the welding station could be reduced by increasing the number of robots, but here again there is a threshold to this possibility due both to economic reasons and to the fact that beyond a given number of robots, each of them becomes a hindrance to the operation of one or more robots adjacent to it.
On the other hand, the time used by each robot to carry out all the welding operations assigned to it is made up not only of the sum of the times required for the various operations, but also of the time each time used to reach the area to be welded, and such time can be quite long especially when the robot has to follow a relatively winding way, dodging any interference both with parts of the structure to be welded and with the various blocking systems engaged thereon.
On the other hand, it should be considered that at the beginning of the use of laser for welding motor-vehicle structures, available laser generators were relatively less efficient and less powerful than those currently available. With first generation laser generators it was anyway necessary to ensure a relatively close position of the laser head carried by the robot with respect to the structure to be welded, so that from this point of view the use of laser was not particularly advantageous with respect to traditional electric spot welding. With currently available laser systems, conversely, new promising solutions open the way to a dramatic reduction of manufacturing costs.
The idea that underlies said evolution and that has been the object of first tests made by the Application, consists in handling the laser head with a robot, holding it at a given distance from the structure to be welded, and in arranging means enabling to orient the focusing direction of the laser beam, so as to shift the latter along the structure to be welded in a direction and at a speed not depending directly on the direction and speed with which the robot shifts the laser head. This is obviously exploited not only for moving the laser beam with respect to the structure to be welded in a given area, in order to obtain a weld seam, but also and especially for welding rapidly and efficiently various areas of the structure to be welded, during the shift of the head carried by the robot.
A solution related hereto has been proposed by the Applicant in European patent application no. EP 1 228 835 A1 (System and method for remote laser welding), which the Applicant is co-owner of. Said known system, however, applies to a “Cartesian” robot and not to an “anthropomorphous” robot, and it is “added” to and not integrated into the robot.
A similar solution, though implemented with a device “integrated” into the robot structure, has further been the object of the previous Italian patent application TO2003A001017 belonging to the same Applicant, filed on Dec. 18, 2003 and still undisclosed at the date on which the present application has been filed. Said solution, as the one disclosed in European patent application EP 1 228 835 A1 mentioned above, is nevertheless quite complex, especially concerning the device for focusing and orienting the laser beam and its control means.